Remove stdatomic.h & tgmath.h

Inspired by picolibc/picolibc@7c9e9c3

As per Keth Packard: "These are provided by the compiler and
shouldn't be replaced with our versions."

And indeed, an attempt to use these Newlib's headers with Clang-based compilers
lead to compilation error:
---------------------->8---------------------
clang -c test.c -I.../newlib/install/arc/include
test.c:8:14: error: '__builtin_choose_expr' requires a constant expression
        f1 = acos(f);
             ^~~~~~~
.../newlib/install/arc/include/tgmath.h:111:20: note: expanded from macro 'acos'
                                ^~~~~~~~~~~~~~~~~~~~
.../newlib/install/arc/include/tgmath.h:107:2: note: expanded from macro '__tg_full'
        __tg_impl_full(__x, __x, __x, __fn, __fn##f, __fn##l, c##__fn, c##__fn##f, c##__fn##l, __x)
        ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.../newlib/install/arc/include/tgmath.h:96:6: note: expanded from macro '__tg_impl_full'
            __tg_impl_simple(__x, __y, __z, __cfn, __cfnf, __cfnl, __VA_ARGS__),        \
            ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.../newlib/install/arc/include/tgmath.h:86:3: note: expanded from macro '__tg_impl_simple'
                __tg_type_corr(x, y, z, double) || __tg_integer(x, y, z),\
                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
.../newlib/install/arc/include/tgmath.h:71:2: note: expanded from macro '__tg_type_corr'
        (__tg_type3(__e1, __e2, __e3, __t) ||                           \
        ^
1 error generated.
---------------------->8---------------------

See foss-for-synopsys-dwc-arc-processors/toolchain#584

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>